The present invention relates, in general, to a loop filter in a phase lock loop (PLL), and more particularly, to such a loop filter responsive to two input current sources to operate in an adapt and a normal mode respectively.
A phase lock loop (PLL) typically includes a loop filter that is operated in an adapt mode and a normal mode. In the adapt mode, the loop filter is designed to have a wide-bandwidth frequency response. The loop filter is operated in the adapt mode for a predetermined period of time so as to enable it to quickly steer an output frequency of the PLL closer to a final value.
After the predetermined period, the loop filter is switched to operate in the normal mode. In the normal mode, the loop filter is designed to have a narrow-bandwidth frequency response for maximum attenuation of spurious signals. U.S. Pat. No. 5,774,023 discloses such a PLL.
Current implementations of the loop filter include a resistor-capacitor (RC) circuit that introduces an additional pole in the normal mode frequency response to make its bandwidth narrow. The RC circuit however causes a problem in the adapt mode. The RC circuit introduces excessive phase shift to produce a low phase margin. A phase margin that is too low indicates that the loop filter is potentially unstable when used in a closed loop mode. The desired wide bandwidth in the adapt mode has to be compromised to increase the phase margin to an acceptable value.
One prior art loop filter includes a switch that is actuatable in the adapt mode to bypass a pole introducing circuit to reduce phase shift caused by the circuit. Such an implementation however requires an additional switch and associated control circuitry.
According to one aspect of the invention there is provided a loop filter having an adapt input, a normal input and an output. Each of said inputs is connectable to mutually exclusively operable current sources to operate said loop filter in an adapt and a normal mode respectively for controlling a voltage controlled oscillator connectable to said output. Said loop filter includes a first circuit which when operated in said normal mode is connected between said normal input and said output to introduce a pole to a normal mode frequency response of said loop filter. When switched to operate in said adapt mode said first circuit becomes connected in series between said output and a signal ground to introduce a zero to an adapt mode frequency response of said loop filter.
Preferably, said normal mode frequency response should include at least one other pole.
Suitably, said loop filter further may include a second circuit connected between said output and said signal ground for introducing said at least one other pole.
Suitably, said first circuit may include an RC circuit.
Suitably, said RC circuit may include a resistor and a capacitor.
Suitably, said resistor is connected in series between said normal input and said output and said capacitor is connected between said normal input and said signal ground.
Preferably, said second circuit should include a second capacitor and a low current damping resistor connected in parallel with said second pole filter capacitor to form an RC parallel connection having two nodes. A first of said two nodes is connected to said output and a second of said two nodes is connected to said adapt input. Said second circuit further includes a loop filter capacitor and a high current damping resistor connected in series with said loop filter capacitor to form a series connection, said series connection being connected between said second of said two nodes and said signal ground.